1. Field of the Invention
The present invention relates to an image forming apparatus such as a color printer, a color copying machine of electrophotographic type and the like.
2. Related Background Art
FIG. 9 shows a conventional color image forming apparatus. A primary charger 651, a plurality of developing devices 652, 653, 654 and 655 containing yellow developer, magenta developer, cyan developer and black developer, respectively, and a cleaner 656 are arranged around a conductive image bearing member 650.
Above the image bearing member 650, there is disposed an exposure means 657 comprising a laser, mirrors and the like. Below the image bearing member 650, there is disposed a transfer means 658. The transfer means 658 comprises a conductive drum body 659, an elastic member 660 made of urethane foam, silicone rubber foam or the like and covering the drum body, and a flexible sheet 661 made of polyethylene terephtalate, polyurethane or the like and covering the elastic member. A gripper 662 for holding a transfer material is provided on the transfer drum. Sheet-shaped transfer materials 664 are stacked in a sheet supply cassette 663.
Around the transfer means 658, there are arranged an adsorb roller 665 for electrostatically absorbing the transfer material 664 to the flexible sheet 661, and a separating charger 666 for separating the transfer material 664 from the flexible sheet 661. The reference numeral 668 denotes a fixing device. Incidentally, the drum body 659 is connected to a transfer charging high voltage source (not shown).
Explaining an image forming process of the above-mentioned color image forming apparatus, first of all, a first color electrostatic latent image formed on the image bearing member 650 by exposing image light A corresponding to a first color image signal from the exposure means 657 on the image bearing member is visualized by the developing device 652 containing the yellow developer, for example.
On the other hand, the supplied transfer material 664 is pinched between the transfer means (transfer material bearing member) 658 and the absorb roller 665 by the absorb roller 665 which can be abutted against and separated from the transfer means 658. At the same time, the absorb and transfer bias is applied to the drum body 659 and the absorb roller 665, with the result that the charge is created to hold the transfer material 664 on the transfer means 658 by the electrostatical absorbing force. Then, the transfer material 664 is conveyed to a transfer station (nip B between the image bearing member 650 and the transfer means), where the first color visualized image formed on the image bearing member 650 is transferred onto the transfer material.
Thereafter, the residual developer remaining on the image bearing member 650 is removed by the cleaner 656, and then, a new electrostatic latent image is formed on the image bearing member 650 by the exposure means 657 in response to a second color image signal. This electrostatic latent image is developed by the developing device 653 containing the magenta developer, for example, thereby forming a visualized image. This second color visualized image is transferred onto the transfer material to which the first color visualized image has already been transferred. Similarly, a third color (cyan) visualized image and a fourth color (black) visualized image are successively formed on the image bearing member 650 by using the cyan developer and the black developer, respectively. These visualized images are successively transferred onto the same transfer material 664 in a superimposed fashion in the same manner as the second color visualized image.
After the transferring operations are finished, the charge on the transfer material 664 is removed by the separating charger 666 to remove the electrostatic absorbing force between the transfer material and the transfer means 658, and then the transfer material is separated from the transfer means by a separating pawl 669. Then, the transfer material 664 is sent to the fixing device 668, where the superimposed images are permanently fixed to the transfer material. When the second color visualized image is transferred, a value of the transfer bias is changed to correct the potential drop after the transferring of the first color visualized image. 0f course, such correction is effected when the third and fourth visualized images are transferred.
By the way, the transfer means 658 of the aforementioned color image forming apparatus is opposed to the image bearing member 650 with a small gap (about 100 .mu.m) or with an appropriate nip. 0n the other hand, during the formation of the image, the image bearing member 650 is charged with the predetermined potential (for example, -600 Volts) by the primary charger 651.
When the electrostatic latent image is not formed on the image bearing member 650, for example, during the pre-rotation of the image bearing member before the latent image is formed or after the visualized image was formed, the image bearing member 650 charged with the potential having the highest absolute value is contacted with the transfer means 658. Consequently, due to the difference in potential between the image bearing member 650 and the transfer means 658, the charges on the surface of the image bearing member 650 are transferred onto the surface of the transfer means 658. Particularly in an image forming apparatus utilizing the reverse image, since the polarity of the primary charge, the polarity of the developer and the polarity of the absorbing bias are the same, the surface of the transfer means 658 is charged to the polarity repelling the developer, by the chargers transferred from the image bearing member 650. As a result, the transfer electric field is weakened by the charge repelling the developer, so that the potential difference sufficient to effect the good transferring and the good absorption cannot be established between the transfer means 658 and the image bearing member 650 and between the transfer means and the absorb roller 665, thereby causing the poor transferring and poor absorpiton.
To eliminate the above problem, it is considered to increase the values of the transfer bias and the absorbing bias by amounts corresponding to the increased charge of the transfer means 658 by the charges transferred from tile image bearing member 650. However, if the value of the transfer bias is increasd to cancel the charge amount transferred from the image bearing member 650, it is feared that the transfer bias exceeds the endurance limit of the flexible sheet 661, thereby creating the pinhole in the flexible sheet, 661. To avoid this, if a thickness of the flexible sheet 661 is increased, since the electrostatic capacity of the transfer means 658 as the thickness of the flexible sheet 661 is increased, the further high transfer bias is required to obtain the effective transfer current.
Further, since the resistance of the flexible sheet 661 is varied from the insulation zone to the intermediate resistance zone in accordance with the environmental condition, under the high temperature and high humidity condition, the increased potential is decreased to some extent during the rotation of the transfer means. However, under the low temperature and low humidity condition, the increased potential is not substantially decreased. Thus, the increasing value of the transfer bias for cancelling the charge transferred from the image bearing member must be changed in accordance with the environmental conditions, with the result that any temperature detection sensor and humidity detection sensor must be provided, which leads to the increase of the number of parts, thereby making the apparatus large-sized and expensive.